HST/WFC3 Grism Observations of

We present and publicly release (www.gclasshst.com) the first spatially resolved Hα maps of star-forming cluster galaxies at z ∼ 1, made possible with the Wide Field Camera 3 (WFC3) G141 grism on the Hubble Space Telescope (HST). Using a similar but updated method to 3D-HST in the field environment, we stack the Hα maps in bins of stellar mass, measure the half-light radius of the Hα distribution, and compare it to the stellar continuum. The ratio of the Hα to stellar continuum half-light radius, , is smaller in the clusters by (6 ± 9)%, but statistically consistent within 1σ uncertainties. A negligible difference in R[Hα/C] with environment is surprising, given the higher quenched fractions in the clusters relative to the field. We postulate that the combination of high quenched fractions and no change in R[Hα/C] with environment can be reconciled if environmental quenching proceeds rapidly. We investigate this hypothesis by performing similar analysis on the spectroscopically confirmed, recently quenched cluster galaxies. 87% have Hα detections, with star formation rates 8 ± 1 times lower than star-forming cluster galaxies of similar stellar mass. Importantly, these galaxies have an R[Hα/C] that is (81 ± 8)% smaller than coeval star-forming field galaxies at fixed stellar mass. This suggests the environmental quenching process occurred outside-in. We conclude that disk truncation due to ram pressure stripping is occurring in cluster galaxies at z ∼ 1, but more rapidly and/or efficiently than in z ≲ 0.5 clusters, such that the effects on R[Hα/C] become observable just after the cluster galaxy has recently quenched.